15  Unit 4: Seasons (Optional 3E)

How does the annual cycle of Earth around the Sun create seasonal temperature variations?

16 Engage: Why Do We Have Seasons?

16.0.1 🤔 Initial Question

Many people believe seasons are caused by Earth being closer to or farther from the Sun. Is this correct?

Fact: Earth is actually closest to the Sun (perihelion) in early January during Northern Hemisphere winter!

16.1 Interactive: Earth’s Orbit Distance

Plot = require("@observablehq/plot")

orbitData = [
  {month: "Jan", distance: 147.1, season: "Winter (NH)"},
  {month: "Feb", distance: 147.8, season: "Winter"},
  {month: "Mar", distance: 148.9, season: "Spring"},
  {month: "Apr", distance: 150.0, season: "Spring"},
  {month: "May", distance: 151.0, season: "Spring"},
  {month: "Jun", distance: 151.8, season: "Summer"},
  {month: "Jul", distance: 152.1, season: "Summer (NH)"},
  {month: "Aug", distance: 151.5, season: "Summer"},
  {month: "Sep", distance: 150.5, season: "Fall"},
  {month: "Oct", distance: 149.3, season: "Fall"},
  {month: "Nov", distance: 148.1, season: "Fall"},
  {month: "Dec", distance: 147.3, season: "Winter"}
]

Plot.plot({
  title: "Earth-Sun Distance Throughout the Year",
  subtitle: "Distance in millions of kilometers",
  width: 650,
  height: 350,
  x: {label: "Month", domain: orbitData.map(d => d.month)},
  y: {label: "Distance (million km)", domain: [146, 153]},
  marks: [
    Plot.line(orbitData, {x: "month", y: "distance", stroke: "#e67e22", strokeWidth: 3}),
    Plot.dot(orbitData, {x: "month", y: "distance", fill: "#e67e22", r: 6}),
    Plot.text([{month: "Jan", distance: 147.1}], {x: "month", y: "distance", text: d => "Closest!", dy: -15, fill: "red", fontWeight: "bold"}),
    Plot.text([{month: "Jul", distance: 152.1}], {x: "month", y: "distance", text: d => "Farthest!", dy: -15, fill: "blue", fontWeight: "bold"})
  ]
})

17 Explore: Axial Tilt and Sunlight

17.1 PhET Simulation: Seasons

Explore how Earth’s axial tilt creates seasons:

Note: If the simulation doesn't load, click here to open it in a new tab.

17.1.1 🔬 Exploration Tasks

1. Observe how the angle of sunlight changes throughout the year at different latitudes
2. Compare day length at 45°N during summer vs. winter solstice
3. Notice how the Sun’s rays are more direct in summer and more spread out in winter

17.2 Interactive: Sunlight Angle Simulator

viewof sunAngle = Inputs.range([0, 90], {
  step: 5,
  value: 45,
  label: "Sun Angle (degrees from horizon):"
})

spreadFactor = 1 / Math.sin(sunAngle * Math.PI / 180)
intensityPercent = (Math.sin(sunAngle * Math.PI / 180) * 100).toFixed(1)

html`<div style="display: flex; gap: 30px; align-items: center; margin: 20px 0;">
  <div style="flex: 1;">
    <svg width="300" height="200">
      <!-- Ground -->
      <rect x="0" y="150" width="300" height="50" fill="#8B4513"/>
      <!-- Sun rays -->
      <line x1="50" y1="${150 - 100 * Math.tan(sunAngle * Math.PI / 180)}" x2="50" y2="150" stroke="yellow" stroke-width="3"/>
      <line x1="100" y1="${150 - 100 * Math.tan(sunAngle * Math.PI / 180)}" x2="100" y2="150" stroke="yellow" stroke-width="3"/>
      <line x1="150" y1="${150 - 100 * Math.tan(sunAngle * Math.PI / 180)}" x2="150" y2="150" stroke="yellow" stroke-width="3"/>
      <!-- Angle arc -->
      <path d="M 50 150 L 50 100 A 50 50 0 0 1 ${50 + 50 * Math.cos((90-sunAngle) * Math.PI / 180)} ${150 - 50 * Math.sin((90-sunAngle) * Math.PI / 180)}" fill="none" stroke="red" stroke-width="2"/>
      <text x="70" y="130" fill="red" font-size="14">${sunAngle}°</text>
    </svg>
  </div>
  <div style="flex: 1; padding: 20px; background: #f5f5f5; border-radius: 10px;">
    <h4>Energy Intensity</h4>
    <p><strong>Relative intensity:</strong> ${intensityPercent}%</p>
    <p><strong>Energy spread factor:</strong> ${spreadFactor.toFixed(2)}x</p>
    <div style="background: linear-gradient(to right, #ffeb3b ${intensityPercent}%, #ccc ${intensityPercent}%); height: 30px; border-radius: 5px;"></div>
    <p style="font-size: 12px; margin-top: 10px;">Higher sun angles = more concentrated energy = warmer temperatures</p>
  </div>
</div>`

18 Explain: The Science of Seasons

18.0.1 🌍 Key Concepts

Seasons are caused by Earth’s 23.5° axial tilt, NOT distance from the Sun.

1. Summer: Your hemisphere is tilted TOWARD the Sun
   • Sun appears higher in the sky
   • Sunlight hits more directly (concentrated energy)
   • Longer days = more heating time
2. Winter: Your hemisphere is tilted AWAY from the Sun
   • Sun appears lower in the sky
   • Sunlight hits at an angle (spread out energy)
   • Shorter days = less heating time

18.1 Connection to Climate Change

Understanding seasons helps us understand climate change because:

• Milankovitch Cycles involve changes to Earth’s tilt over thousands of years
• These orbital changes have driven glacial-interglacial cycles
• We’ll explore this in detail in the next lesson!

18.1.1 ✅ Check Your Understanding

1. Why is it WARMER in summer even though Earth is FARTHER from the Sun?
2. How does the angle of sunlight affect heating?
3. Why do the Northern and Southern hemispheres have opposite seasons?

---

18.2 📝 Seasons Quiz

Question 1: Earth is closest to the Sun during which month? - A) July - B) January - C) March - D) September

Question 2: What primarily causes Earth’s seasons? - A) Distance from the Sun - B) Earth’s axial tilt of 23.5° - C) Changes in the Sun’s output - D) Ocean currents

Question 3: During Northern Hemisphere summer, the North Pole is: - A) Tilted away from the Sun - B) Tilted toward the Sun - C) Not tilted at all - D) Experiencing 24 hours of darkness

Answers: 1-B, 2-B, 3-B